1. Field of the Invention
The present invention relates to the field of information recording media, and more particularly relates to the field of optically readable recording media, such as optical discs.
2. Background Art
An optically readable storage medium is one which can be read by way of a beam of light which is imaged to a small spot on the medium. One common form of optically readable recording medium is the optical disc. Optical discs are in increasing use for the recording and playback of video program information.
The most widespread form of optical disc comprises a flat disc made of two disc-shaped parts of transparent material which are separately constructed and then glued together to form the disc. This two-sided construction permits the disc to carry two sets of stored information, one on each disc part. The disc is flat, circular and approximately the size of an LP phonograph record.
Information is stored on a side of a disc in uniform circular or spiraling tracks of optically readable indicia. The indicia are readable by virtue of exhibiting optical contrast with respect to the surrounding surface in which they are formed. For example, a spot of light imaged onto the recording surface of a disc side at a location where no indicia are present may have most of its light reflected directly back in mirror-like reflection. However, a spot of light imaged onto one of the indicia may be scattered, thus resulting in less light being reflected directly back from the recording surface. By arranging indicia in row-like tracks, a spot of light can be scanned along a track and the variation in the intensity of the light reflected directly back may be detected to extract the information from the disc.
Commercial optical discs generally take a form in which the flat inner surface of each disc part is used as a recording surface in which the tracks of indicia are molded in the form of tiny pits. Since each disc part is read by imaging the spot of light through the disc from the side opposite the recording surface, the tiny pits on the recording surface are presented to the imaged beam of light as a series of bumps.
The amount of information that can be stored on an optical disc depends upon the packing density of the indicia. Consequently, tracks and the indicia within them are spaced as close together as possible. A common distance selected by optical disc manufacturers for track-to-track spacing, for example, is approximately 1.6 microns. This track spacing permits the optical disc to be played while the imaged spot of light scans along the track, wobbling back and forth within the limits of the range of the servo control mechanism of the disc player, while maintaining the influence of adjacent tracks, called "cross-talk", to an acceptable minimum for video program playback. Closer track-to-track spacing in this format has been found to result in a rapidly increasing level of cross-talk to unacceptable levels. In addition, while the level of cross-talk which can be achieved using the format just described is generally considered to provide satisfactory playback performance, it is desired to improve the level of cross-talk suppression even more.
Accordingly, it will be appreciated that there is a need for an improved format for an optically readable recording medium which provides reduced cross-talk between adjacent tracks. In addition, there is a need for an optically readable recording medium which provides improved track-to-track spacing while maintaining acceptable levels of cross-talk between tracks. The present invention satisfies these needs.